Condition等待唤醒

考虑考虑2024-07-13 17:38

前言

在Java中，每个对象都可以调用 Object 的 wait/notify 方法来实现等待/通知机制。而 Condition 接口也提供了类似的方法，也能实现等待唤醒，Condition需要在ReentrantLock下使用，也就是需要上锁和释放锁，否则也会报java.lang.IllegalMonitorStateException

Condition使用

Condition方法

Condition提供await()、signal()、signalAll()方法，其中，await()是让线程等待，signal()是唤醒一个线程，signalAll()是唤醒全部线程,当前线程调用condition.await()方法后，会释放 lock 然后加入到等待队列，直到被 signal/signalAll 方法唤醒

Condition等待唤醒使用

1、await()等待唤醒

ini 复制代码 
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class ConditionTest {

    public static void main(String[] args) {
        final ReentrantLock lock = new ReentrantLock();
        final Condition condition = lock.newCondition();

        Thread thread1 = new Thread(() -> {
            String name = Thread.currentThread().getName();

            lock.lock();
            try {
                System.out.println(name + " <==进入条件队列等待");
                condition.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.unlock();
            System.out.println(name + ":线程被唤醒");
        }, "aa");

        thread1.start();

        Thread thread2 = new Thread(() -> {
            String name = Thread.currentThread().getName();

            lock.lock();
            try {
                Thread.sleep(5000);
                System.out.println(name + ":开始唤醒线程");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            condition.signalAll();
            lock.unlock();
            System.out.println(name + ":唤醒结束");
        }, "通知线程");

        thread2.start();
    }
}

输出结果为

signal()只能唤醒一个线程

ini 复制代码 
package cn.com.ut.july.wait;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class ConditionTest {

    public static void main(String[] args) {
        final ReentrantLock lock = new ReentrantLock();
        final Condition condition = lock.newCondition();

        Thread thread1 = new Thread(() -> {
            String name = Thread.currentThread().getName();

            lock.lock();
            try {
                System.out.println(name + " <==进入条件队列等待");
                condition.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.unlock();
            System.out.println(name + ":线程被唤醒");
        }, "aa");

        thread1.start();

        Thread thread3 = new Thread(() -> {
            String name = Thread.currentThread().getName();

            lock.lock();
            try {
                System.out.println(name + " <==进入条件队列等待");
                condition.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.unlock();
            System.out.println(name + ":线程被唤醒");
        }, "aa1");

        thread3.start();

        Thread thread2 = new Thread(() -> {
            String name = Thread.currentThread().getName();

            lock.lock();
            try {
                Thread.sleep(5000);
                System.out.println(name + ":开始唤醒线程");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            condition.signal();
            lock.unlock();
            System.out.println(name + ":唤醒结束");
        }, "通知线程");

        thread2.start();
    }
}

输出结果为

signalAll()唤醒全部线程

ini 复制代码 
package cn.com.ut.july.wait;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class ConditionTest {

    public static void main(String[] args) {
        final ReentrantLock lock = new ReentrantLock();
        final Condition condition = lock.newCondition();

        Thread thread1 = new Thread(() -> {
            String name = Thread.currentThread().getName();

            lock.lock();
            try {
                System.out.println(name + " <==进入条件队列等待");
                condition.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.unlock();
            System.out.println(name + ":线程被唤醒");
        }, "aa");

        thread1.start();

        Thread thread3 = new Thread(() -> {
            String name = Thread.currentThread().getName();

            lock.lock();
            try {
                System.out.println(name + " <==进入条件队列等待");
                condition.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.unlock();
            System.out.println(name + ":线程被唤醒");
        }, "aa1");

        thread3.start();

        Thread thread2 = new Thread(() -> {
            String name = Thread.currentThread().getName();

            lock.lock();
            try {
                Thread.sleep(5000);
                System.out.println(name + ":开始唤醒线程");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            condition.signalAll();
            lock.unlock();
            System.out.println(name + ":唤醒结束");
        }, "通知线程");

        thread2.start();
    }
}

输出结果为

Condition实现三个线程顺序输出

scss 复制代码 
package cn.com.ut.july.wait;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class ConditionDemo {

    public static void main(String[] args) throws InterruptedException {

        ReentrantLock lock = new ReentrantLock();
        Condition condition1 = lock.newCondition();
        Condition condition2 = lock.newCondition();
        Condition condition3 = lock.newCondition();

        new Thread(() -> {
            while (true) {
                try {
                    Thread.sleep(500);
                    System.out.println("A");
                    lock.lock();
                    condition2.signal();
                    condition1.await();
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                } finally {
                    lock.unlock();
                }
            }
        }).start();
        Thread.sleep(100);
        new Thread(() -> {
            while (true) {
                try {
                    Thread.sleep(500);
                    System.out.println("B");
                    lock.lock();
                    condition3.signal();
                    condition2.await();
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                } finally {
                    lock.unlock();
                }
            }
        }).start();
        Thread.sleep(100);
        new Thread(() -> {
            while (true) {
                try {
                    Thread.sleep(500);
                    System.out.println("C");
                    lock.lock();
                    condition1.signal();
                    condition3.await();
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                } finally {
                    lock.unlock();
                }
            }
        }).start();

    }

}

输出结果为

使用Thread.sleep()是为了保证先start()的线程先启动，以及不输出那么快

总结

Condition的await()和signal()基于Lock，相比于基于Object的wait()和notify()方法，它提供更加灵活的等待通知的机制，并且是基于AQS队列形式，但是在开发过程中，根据自己喜好选择

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